1. Field of the Invention
The invention relates to a thin film transistor manufacturing method and an organic EL display device. This thin film transistor manufacturing method can be applied to a manufacturing method of a driving transistor of an organic electroluminescent element, for example.
2. Description of the Related Art
In recent years, an organic electroluminescent (hereafter, referred to as EL) display device with an organic EL element has been gathering attention as a display device replacing a CRT and an LCD. Particularly, the development efforts for the organic EL display device with a thin film transistor (referred to as TFT, hereafter) as a switching element for driving the organic EL element has been made accordingly.
An equivalent circuit diagram of an organic EL display panel 300 is shown in FIG. 8. A plurality of gate signal lines 11 supplying a gate signal and a plurality of drain signal lines 12 supplying a display signal Vsig are crossing each other. Pixels are disposed at intersections of both the signal lines, forming a display screen 10. Each of the pixels includes a pixel selecting TFT 13, an organic EL element 14, a driving TFT 15 driving this organic EL element 14, and a storage capacitor 16 temporarily storing the display signal Vsig.
A gate of the pixel selecting TFT 13 is connected with the gate signal line 11 and supplied with a gate signal, and a drain thereof is connected with the drain signal line 12 and supplied with a display signal Vsig. A source of the pixel selecting TFT 13 is connected with a gate of the driving TFT 15. The gate signal is outputted from a vertical drive circuit 30. The display signal Vsig is supplied through a horizontal drive circuit 20.
A source of the driving TFT 15 is supplied with a positive potential PVdd, and a drain thereof is connected with an anode of the organic EL element 14. A cathode of the organic EL element 14 is supplied with a negative potential CV.
An operation of the organic EL display device having the above-described structure will be described. When a gate signal becomes high level for a horizontal period, the pixel selecting TFT 13 turns on. Then, a display signal Vsig is applied from the drain signal line 12 to the gate of the driving TFT 15 through the pixel selecting TFT 13.
Then, the conductance of the driving TFT 15 changes in response to the display signal Vsig supplied to the gate, a corresponding amount of drive current is supplied to the organic EL element 14 through the driving TFT 15, and the organic EL element 14 turns on a light. When the driving TFT 15 turns off in response to a display signal supplied to the gate, an electric current does not flow in the driving TFT 15, so that the organic EL element 14 turns off the light.
The relevant technology is disclosed in the Japanese Patent Application Publication No. 2002-175029.
In the described organic EL display panel 300, however, as shown in FIG. 7, there has been a problem that a horizontal streak (a streak pattern in a horizontal direction on the screen) or a vertical streak (a streak pattern in a vertical direction on the screen) appears on the display screen 10 as display unevenness, or “mura” as known in the art.
The inventor founds that a cause of the appearance of the horizontal streak or the vertical streak on the display screen 10 is variation in the drive current driving the organic EL element 14 among the driving TFTs 15, and such variation in the drive current occurs by variation in energy of excimer laser irradiation within the display screen 10, which is performed when an amorphous silicon film forming the driving TFT 15 is crystallized.